This invention relates to a novel composite material, and more particularly to a composite material for construction structures which has excellent strength, heat resistance and alkali resistance and which is suitable for use as substitutes of reinforcing steel bars for concrete structures or of strands for prestressed concrete structures.
Generally, concrete structures are reinforced by steel bars. In recent years, sea sand having a high salinity has been increasingly used in place of pit sand in preparing a concrete. However, the salinity gradually corrodes the reinforcing steel bars to deteriorate its strength and thus the durable period is reduced.
There is a continued need to use substitutes for concrete reinforcement in place of the reinforcing steel bars. In recent years, substitutes have been developed. It is desirable that the substitutes exhibit the following characteristics. First, the substitutes have high alkali resistance such that they withstand an alkali in a concrete. Secondly, the substitutes have high strength, and preferably have light weight and high strength. Thirdly, the substitutes have high heat resistance, and thus they neither ignite nor burn even if the concrete structure is surrounded by a severe flame and the explosive fracture of the structure occurs eventually to expose the substitutes.
This is also true in the cases of strands for prestressed concrete structures or substitutes thereof. The foregoing is also true in the case of substitutes for steel frames. Interior finishing materials and exterior finishing materials having durability and light weight are demanded.
In recent years, fibers having high performance and high strength such as carbon fibers have been developed. Such fibers have been used in the field of a construction in order to obtain fiber-reinforced composite materials wherein they have light weight, good workability and productivity, wherein the weight of the structure can be reduced and wherein durability can be further improved. However, the fiber-reinforced composite materials heretofore proposed lack heat resistance and the like, they do not combine the characteristics described above and they cannot be used in general practice.
For example, Japanese Patent Laid-Open Pub. No. 146024/1989 discloses a concrete structure obtained by forming fibers such as carbon fibers and glass fibers with resins such as vinyl ester resins and epoxy resins to make a material and using the material as a substitute of a reinforcing steel bar. However, such a material has insufficient heat resistance. For example, when the material is exposed to high temperatures of at least 300.degree. C., it ignites and gradually becomes porous. Thus, this allows the combustion of a volatile matter of resins, and carbonization occurs. Accordingly, the material does not satisfy the requirements of a building code. Such a material is only applied to structures for civil engineering requiring less heat resistance.
Japanese Patent Laid-Open Pub. Nos. 215634/1986 and 121450/1989 disclose concrete-reinforcing materials obtained by forming carbon fibers with resins such as furan-modified epoxy resins. However, these materials lack heat resistance and exhibit insufficient alkali resistance.